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Help: how to design air-core transformer?

Discussion in 'Electronic Components' started by Jim, Nov 22, 2004.

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  1. Jim

    Jim Guest

    I hope to design an air-core transformer which may operate at 100-500 MHz.
    However I found the loss is too high.
    Can anybody give me some advices to design air-core transformer?
    Thanks a million!
  2. James Meyer

    James Meyer Guest

    It can't be done. Try another solution.

  3. James Meyer wrote...
    Like a transmission-line transformer, which can even be made
    without ferrites, if you sacrifice the low end.
  4. Leon Heller

    Leon Heller Guest

    RF inductors are often wound without cores, especially at VHF and UHF.

  5. legg

    legg Guest

    What is your loss like WITH a core? Maybe the core is not your
    problem. What application/environment?

    Google 'air-core transformer', 'air cored transformer', 'coreless
    transformer', 'contact-free transmission', 'contactless transmission',
    'transcutaneous energy' etc.

  6. Leon Heller wrote...
    Right. But the output of a transformer (two coupled inductors)
    ends up being controlled more by coupled capacitance than by
    magnetic field at those frequencies. Hmm, perhaps if one were
    to interpose a shield... with approriate slits... Hmm...
  7. James Meyer

    James Meyer Guest

    Low loss wideband RF transformers are almost never wound without cores.

  8. Exactly. A common practice is to wind the windings by twisting them
    together, bifilar or trifilar, etc. This increases the coupling. At
    several hundreds of MHz, the turns may be just a few or one turn.
    Here's a tutorial on how to wind one.
  9. Watson A.Name - \"Watt Sun, the Dark Remover\" wrote...
    But that's with cores. If there's no core, and the length is made
    long enough for the desired response, you'll end up primarily with
    a coupling capacitor. Or a transmission line.
  10. Marc H.Popek

    Marc H.Popek Guest

    There is a very good Motorola applications note discussing wide band baluns,
    transformers, etc. both ferrite wound and air wound..

    The treatment of the structure as a transmission line is appropriate. Actual
    coax and the bi-filar and tri-filar wires are used. So how wide a band
    width are you going to use?


    RF power and components FA here
  11. Leon Heller

    Leon Heller Guest

    I was thinking of a narrow-band transformer within the range the specified

  12. Jim

    Jim Guest

    I observed two unwanted phenomenon:

    1. direct capacitive coupling between primary/secondary coils. It
    interferes with the mutual inductance, and makes the transmissivity of
    the transformer change significantly with frequency.

    2. Insertion loss is higher than 10dB.

    I googled air core transformer before i posted my question. It is
    weird that the concept is mentioned in many places, but I can never
    find a real one which works in VHF/UFH band except transmission-line

    My purpose is for impediance matching, from 50 ohm cable to a 0.5 ohm

    To my understanding, air-core transmission-line transformers have
    fixed conversion ratio, e.g. 1:1, 4:1 or 1:4, is it right? What i need
    is a ratio of 10:1.

    I saw some companies provide RF transformer with ferrite coil inside.
    The insertion loss is only 1-2dB, however, for UHF band, the maximum
    ratio is 4:1 or 6:1 only.

    Thank you all again! And hope to hear from you again.
  13. Jim wrote...
    You could simply put a 50-ohm resistor in series with your
    o.5-ohm load, to prevent reflections and insure a fixed
    known current at all operating frequencies.

    As far as wideband transformers are concerned, the common
    4:1 transmission-line transformers (get Sevick's books) can
    be made for 50 to 12.5 ohms and then 12.5 to 3.25 ohms. The
    transmission line in each stage must have an impedance that's
    the geometric mean between Zin and Zout, i.e. 25 ohms and
    6.5 ohms. You'll have to make the transmission line yourself.
    I handmade a 16:1 wideband transformer this way, and it was
    a tricky task, taking several days. But it worked well.

    In theory you can do the next step from 3.25 to 0.81 ohms,
    but it's nearly impossible to make 1.6-ohm transmission line.

    Perhaps the spot for a resistive match is here, at the 3.25
    to 0.5-ohm level. At least you will have quadrupled the
    delivered current into 0.5 ohms with your 16:1 transformer.

    If you want to play a narrow-band matching game, no problem,
    have at it.
  14. legg

    legg Guest

    The old Motorola App Note AN1304 discusses the use of coaxial baluns
    around 1GHz, to reduce Z mismatches of 100:1 to approximately 6:1.


    ......don't forget to check references listed in tha articles.

    Going to 50ohms in the first place, may be part of the problem, as
    your active devices likely exhibit output impedances in the 10ohm
    range or less, to begin with. Matching earlier may help.

    If galvanic isolation is required, a simpler 1:1 or 2:1 conventional
    balun transformer might be introduced additionally, in a convenient
    position. because required turns are low, you might consider winding
    shapes that are not normally seen in baluns - perhaps a single loop
    inside an isolated screen (~output winding).

    With the app data aimed at achieving 50 ohms, it may be difficult to
    turn it around to make sense for low load values such as 0.5 ohms.

  15. budgie

    budgie Guest

    It isn't clear (to me, anyway) why you are specifically seeking an air-cored
    solution. What is the problem with a ferrite core? You can get a very good
    match with a 3:1 turns ratio, or if you really want to get precise then increase
    the turns so you get M:N ratio closer to SQRT(10).
  16. I agree about the ferrite, but he needs a 100:1 impedance change so a
    10:1 turns ratio.
  17. Jim

    Jim Guest

    If I simply need a narrow-band transformer, e.g. 300MHz with
    bandwidth>1MHz, what is the best method? I don't need continuous
    tuning, so I can make a set of such transformers to cover the range of
  18. [snip]

    Looks to me like you need more lie a hundred to one.

    You can do 9:1 or 16:1 with bifilar wound air core coils.
  19. Pig Bladder

    Pig Bladder Guest

    Because that's what the assignment was.
  20. Rhett Oracle

    Rhett Oracle Guest

    The "best method," of course, is the one that yields the most nearly ideal

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